e an accelerating cause which covers
all the loss from tidal action, and leaves a balance of 4/5 a second per
annum in the way of acceleration.

If tidal retardation can be thus checked and overthrown by other
temporary conditions, what becomes of the confident assertion, based
upon the assumed uniformity of tidal retardation, that ten thousand
million years ago the earth must have been rotating more than twice as
fast as at present, and, therefore, that we geologists are "in direct
opposition to the principles of Natural Philosophy" if we spread
geological history over that time?

II. The second argument is thus stated by Sir W. Thomson:--"An article,
by myself, published in 'Macmillan's Magazine' for March 1862, on the
age of the sun's heat, explains results of investigation into various
questions as to possibilities regarding the amount of heat that the sun
could have, dealing with it as you would with a stone, or a piece of
matter, only taking into account the sun's dimensions, which showed it
to be possible that the sun may have already illuminated the earth for
as many as one hundred million years, but at the same time rendered it
almost certain that he had not illuminated the earth for five hundred
millions of years. The estimates here are necessarily very vague; but
yet, vague as they are, I do not know that it is possible, upon any
reasonable estimate founded on known properties of matter, to say that
we can believe the sun has really illuminated the earth for five hundred
million years."[59]

I do not wish to "Hansardize" Sir William Thomson by laying much stress
on the fact that, only fifteen years ago, he entertained a totally
different view of the origin of the sun's heat, and believed that the
energy radiated from year to year was supplied from year to year--a
doctrine which would have suited Hutton perfectly. But the fact that so
eminent a physical philosopher has, thus recently, held views opposite
to those which he now entertains, and that he confesses his own
estimates to be "very vague," justly entitles us to disregard those
estimates, if any distinct facts on our side go against them. However, I
am not aware that such facts exist. As I have already said, for anything
I know, one, two, or three hundred millions of years may serve the needs
of geologists perfectly well.

III. The third line of argument is based upon the temperature of the
interior of the earth. Sir W. Thomson refers to certain inve